Cellular based communications systems and method of using same

ABSTRACT

An apparatus and method for transmitting serial digital data across wireless modems is disclosed. In one embodiment, data is collected by a remote monitoring device and transmitted to a first Cellular Digital Packet Data (“CDPD”) modem via one or more RS232 interfaces. The applications program in the first CDPD modem transmits a signal to a second CDPD modem initiating communications. The applications program in the second CDPD modem returns a signal, indicating the status of the interface. Data transmission is initiated when the status of both interfaces is established.

TECHNICAL FIELD

The present invention relates generally to a communications system and,more specifically, to an apparatus and method for implementing acommunications interface protocol using existing Cellular Digital PacketData (“CDPD”) transmission equipment.

BACKGROUND

Cellular Digital Packet Data (“CDPD”) is a wireless communicationsprotocol that folds streams of data into envelopes or packets that aretransmitted at very high speeds during pauses in cellular phoneconversations. This permits the use of existing cellular systems as ameans of data transmission. CDPD allows data files to be assembled intopackets and transmitted via idle channels of existing bandwidth. Datacan be transmitted at 19.2 Kbps over an enhanced cellular network.Adding CDPD to an existing analog cellular system allows cellularsystems to transmit data eight times faster without the necessity ofcreating a completely new digital system.

In practice, packet data is transmitted in a wireless mode using theavailable bandwidth the Advanced Mobile Phone Service (“AMPS”) whichoperates as the communications infrastructure for analog cellular radio.Digital cellular is referred to as D-AMPS. CDPD specifications arepublished through the CDPD Forum and follow OSI (Open SystemsInterconnection Protocol) guidelines. CDPD technology providesconnectivity up to the network layer and is an overlay system thatoperates on AMPS frequencies.

The RS232 serial interface provides a serial data connection between twodevices over dedicated wires. The interface defines up to 26 linesbetween two devices. One line carries the data and the others carrysignaling information. Signaling is achieved by the lines through binarystates, either “ON” or “OFF.” Some lines are defined for data and somefor signaling. Thus, data transmission can be controlled simultaneous byboth the sending device and the receiving device.

For example, the sending device can query the receiving device as towhether it is ready to receive data by setting the signal high on a linecalled Ready To Send (“RTS”). The receiving device can, in turn, replythat it is ready to receive data by setting the signal high on the ClearTo Send (“CTS”) line. After these conditions are satisfied, by devicescan begin data transmission over the data lines a communication speeds.

SUMMARY OF THE INVENTION

As noted above, wired RS232 applications conduct data communicationsbetween a sender and a receiver that are hard wired together. In a wiredRS232 connection based application, several lines may simultaneously beused to send signaling information; the signals on each line aredetected by the sender and receiver instantaneously. On the other hand,in wireless RS232 communications, instantaneous communication does notoccur. Rather, information is transmitted with a delay of ranging from0.5 to 4 or 5 seconds. Furthermore, in wireless communications, modemscan connect to a large number of devices using the call establishmentmethodology programmed into the modems.

The present invention provides a method and apparatus for using wirelessmodems in an RS232 mode, that provide (1) logic establishing the senderand receiver and (2) a signaling and data transmission methodology thatmakes the wireless mode transparent to the applications. The method andapparatus of the present invention allows utilization of wired RS232based applications transparently on wireless communication systems, andalleviates the delay inherent in wireless communications withoutimpacting the reliability of the system.

The invention provides a wireless point-to-point communications systemfor reliable and efficient digital data transfers as compared to priorart network interface protocols. In this regard, and in accordance withone embodiment, the invention utilizes a commonly availablecommunications protocol, such as RS232, or other accepted serialstandard to encapsulate digital data derived from an instrument, datacollector, or other signal acquisition means in a wireless signalcarrier. The serial protocol has built-in error correction and flowcontrol. The end-to-end interface is compatible with ordinary devicessupporting the protocol. In addition, the cellular communicationsinfrastructure provides a reliable backbone with call switching androuting of the data to its intended destination. The RS-232 data andflow control signal are encapsulated in a CDPD courier supported inexisting cellular infrastructures. Since RS-232 is widely accepted andsupported by a plurality of data acquisition processing systems, theinterface protocol provided is reliable and efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a schematic illustration of a system utilizing standard,wired RS232 data transmission;

FIG. 1(b) is a schematic illustration of a system employing the wirelessdata transmission of the present invention;

FIG. 2 is an illustration of components of a CDPD utilized in thepractice of the present invention;

FIG. 3 is a tabular illustration of the configuration of theconfiguration of two of the lines of the serial data transmission deviceof the present invention;

FIG. 4 is an illustration of the timing of the sequence of steps in datatransmission utilizing the present invention;

FIGS. 5(a) and 5(b) are a flowchart for the transmitting applicationsdriver utilized in the practice of the present invention;

FIGS. 6(a) and 6(b) are a flowchart for the receiving applicationsdriver utilized in the practice of the present invention;

FIGS. 7(a) through 7(e) are a flow chart for control of the initiatingCDPD modem as utilized in the practice of the present invention; and,

FIGS. 8(a) through 8(d) are a flow chart for control of the receivingCDPD modem as utilized in the practice of the present invention.

DETAILED DESCRIPTION

Referring now to FIG. 1(a), a system utilizing standard RS232 datatransmission is illustrated. In the illustrated embodiment, a monitoringdevice 10 retrieves information from a remote device , such as meter 12.The information is normally collected as an analog signal that isconverted to a digital format and subsequently transmitted via wiredRS232 interfaces 14 to a computing device 16 such as a computer,programmable logic controller or data concentrators/multiplexers forstorage and processing.

FIG. 1(b) illustrates a data transmission system utilizing the presentinvention. Data is retrieved from remote device 12 by monitoring device10 and transmitted to a CDPD modem 18 via RS232 interfaces 14. CDPDmodem 18 transmits the information via cellular radio to a second CDPDmodem 18′ which, in turn, transmits the information via RS232 interfaces14 to computing device 16.

Referring now to FIG. 2, three components of the CDPD modem 18 areschematically illustrated. The CDPD modem 18 includes a SubscriberIdentity Module (“SIM”)¹ 20, a Mobile Application Subsystem (“MAS”)² 22and a Subscriber Unit (“SU”)² 24. The mobile application substation 22interfaces with the end user application and the subscriber unit 24manages the air interface. In some applications, the SIM is a “smart”card that is inserted or installed in a cellular phone that contains allsubscriber-related information. The SIM facilitates communications froma valid cellular telephone because the subscriber data is used tocomplete telephone communications rather than the internal serial numberof the telephone. In one embodiment, the present invention provides anenhancement to the SIM with a standard RS232 interface that increasesthe distance that digital data can reliably and accurately betransmitted via cellular communications and supports multiple standardswithout modification.

FIG. 3 is a tabular illustration of the configuration of two of theRS232 interface lines or pins utilized to implement data transmission inaccordance with the present invention. In order to initiate datacommunications, the initiating application sets a “high” signal on afirst line 26. The program in the modem on the initiation side of thetransmission decodes the signal and sets the status of the line in theline entry of the table to “SEND” and the value of the line to “HIGH.”

The entire table 26 is transmitted by the modem 18 on the initiatingside as one packet of data. When the receiving modem receives thepacket, it changes the status of the entry for the first line in thetable to “established” on the RS232 interface. The receiving applicationmust then acknowledge the signal by setting the second line 28 “HIGH” inthe same sequence, i.e., set the status, and transmit the table as onepacket to the receiving modem. The initiating application then sets thestatus to “ESTABLISHED” on the RS232 interface. When both lines are seenas high and established by the initiating application, the initiatingmodem starts data transmission.

The data to be transmitted is concatenated to a signaling packet on theinitiating side and the packet is transmitted. Since both lines are“HIGH” and “ESTABLISHED,” the receiving modem accepts the data. ACyclical Redundancy Check (“CRC”) is used by the receiving modem toconfirm the validity of the data. If the data passes the CRC check, itis accepted as valid. Since the wireless modems 18 and 18′ providewireless error correction, duplication of this function is not required.Incorporation of the CRC, however, makes error checking consistentbetween a wired and wireless application. Through the combination of aminimal logic set and a minimal set of signaling data as set forth abovethe present invention provides transparency of the wireless link to theapplication, the same reliability as a wired link for the applicationand a minimal program size that is important for such applications.

Referring now to FIG. 4 the timing of the sequence of steps in datatransmission utilizing the present invention is schematicallyillustrated. First, the RTS line is pulled “HIGH” on the RS232 interfaceon the initiating side. Next, the CTS line on the RS232 interface ispulled “HIGH” after receipt of the table 26. After the table 26 has beentransmitted back to the initiating modem 18 as described above, datatransmission commences. Upon successful completion of data transmission,i.e., when the receiving application detects the end of text in thedata, it sets the signal “LOW” on the CTS line, and initiates the samesequence of signaling to de-establish the lines.

FIGS. 5(a) and 5(b) are a flowchart for the applications driver for theinitiating modem. As illustrated, when the application is stated thefirst step is to check the status of the RS232 interface (step 102). Ifthe port is busy, a busy signal is returned to the application (step104). If the port is not busy, the application checks to determinewhether the RTS line is high (step 106). If the RTS line is set high, abusy signal is returned to the application (step 108). If the RTS lineis not set high, the port status is set busy (step 110) and the RTS lineis set high and table 30 is transmitted as described above (step 112). Atimer is initiated (step 114). If the timer times out before receipt ofthe table 26 from the receiving application, the port status is set to“ERROR” and the error status is returned to the application (steps 116and 118). If the table 26 is received by the initiating application withthe CTS line set high, the timers are reset (step 124) and datatransmission is initiated.

When the receiving application detects the end of text in the datatransmission it initiates a timer (step 128). If the applicationreceives a CTS “LOW” before the timer times out, the timers are reset(step 130) and the RTS line is set low (step 132). If the timer timesout before the CTS “LOW” is received, the port status is set to “ERROR”(step 134) and the error status is returned to the initiatingapplication (step 136). Assuming that the RTS “LOW” signal is confirmedby the initiating device before the timer is timed out, the port statusis set open (step 138) and a signal indicating successful transmissionis transmitted to the initiating application (step 140). If the RTS lowis not confirmed prior to the timer timing out, the port status is setto “ERROR” (step 142) and the error status is transmitted to theinitiating application (step 144).

Referring now to FIGS. 6(a) and 6(b) a flowchart for the applicationsdriver for the receiving application is illustrated. After theapplication is started, the status of the port is checked (step 200) andif the port is busy a return busy signal is returned to the application(step 202). If the RTS line is set low, a return “IDLE” signal isreturned to the application (step 204) If the RTS line is set “HIGH” theport status is set “BUSY” (step 206), the CTS line is set high (step208), and table 26 is transmitted to the initiating application. A timeris initiated (step 210) and if a data transmission is detected on theport before the timer times out, the data is read (step 214) and thetimer reset (step 216). If no data transmission is detected on the portbefore the timer times out, an error signal is returned to theapplication (step 212).

After the receiving application detects the end of text in the data, itsets the signal in the CTS line “LOW”, transmits the data in table 26(step 218) as described above to the initiating application and sets atimer (step 220). If the receiving application receives confirmationthat the RTS line is set low prior to timing out, the port status is setas “OPEN” (step 222) and a successful data receive signal is returned tothe initiating application (step 224). If confirmation is not received,and if the timer has timed out, the port status is set to “ERROR” (step226) and a return error signal is transmitted to the initiatingapplication (step 228).

Referring now to FIGS. 7(a), through 7(e) a flow chart for the CDPDmodem control for the initiating modem is illustrated. The status of theRTS line is checked (step 300), and if the RTS line is set high thestatus of the RTS line is mapped as “SEND” in the application buffer(step 302) and the application buffer is copied to the communicationstransmit buffer (step 304). The transmit buffer is then transmitted tothe receiving modem as a UDP (“User Data Protocol”) message, (step 306)and a timer is set (step 308). If a message is received from thereceiving modem in the receive buffer (step 310), the receive buffer iscopied to the application buffer (step 312) and the timer is reset (step314). If the timer times out prior to the receipt of a message from thereceiving modem (step 316) the number of attempted transmissions iscompared to a predetermined maximum number of transmissions (step 318).If the number of attempted transmissions exceeds the predeterminedmaximum an error message is returned to the application (step 320).

After the receive buffer is copied to the application buffer, the statusof the RTS and CTS lines in the application buffer is checked (step322). If the RTS line status is set to “ESTABLISHED” and the CTS linestatus is set to “SEND”, the predetermined number of permissibletransmission attempts is reset (step 324) and the CTS line is set high(step 326), otherwise control is returned to step 306. Upon receipt ofdata, the data is read from the data line into the application buffer(step 328) and the CTS line status is set as “ESTABLISHED” in theapplication buffer (step 330). The application buffer is copied to thetransmit buffer (step 332 ), transmitted as a UDP message (step 334) anda timer is set (step 336).

If a message is received from the receiving modem in the receivingbuffer (step 338), before the timer times out, the contents of thereceiving buffer are copied into the applications buffer (step 340). Ifthe timer times out prior to the receipt of a message from the receivingmodem (step 342), the number of attempted transmissions are compared toa predetermined maximum number of transmissions (step 344).

If the number of attempted transmissions exceeds the predeterminedmaximum, an error message is returned to the application, otherwisecontrol is returned to step 334. After the contents of the receivingbuffer is copied into the applications buffer (step 340), the timer isreset and the line status in the application buffer are checked for RTS“ESTABLISHED” and CTS “LOW” (step 344). If these conditions are met, theCTS line is set low (step 346). If the line status in the applicationbuffer for RTS is not “ESTABLISHED” or if CTS is not set low, control isreturned to step 334.

After the CTS line is set low in step 346, the RTS line is checked for a“HIGH” condition (step 348) and the maximum number of permissibleattempted number of attempted transmissions is reset (step 350). If theRTS is “LOW,” control is returned to step 348. After the number ofpermissible attempted transmissions is reset in step 350, the linestatus for RTS in the applications buffer is set low (step 352). Theapplications buffer is copied to the transmit buffer (step 354), thetransmit buffer is transmitted as a UDP message (step 356) and a timeris set (step 358).

If a message is received from the receiving modem in the receivingbuffer (step 360) prior to the timer timing out (step 362), the contentsof the receiving buffer are copied into the application buffer (step364) and the timer is reset (step 366). If the timer times out (step 362) the number of attempted transmissions is compared to the maximumpermissible number (step 368). If the number of attempted transmissionsexceeds the maximum permissible number, an error message is returned tothe application, (step 370); if not, control is returned to step 356.

After the timer is reset in step 366, the line status in theapplications buffer is checked for RTS “LOW” and CTS “LOW” (step 372).If these conditions are not met, control is returned to step 354; if theconditions are met, a signal indicating a successful transmission isreturned to the application (step 374).

Referring now to FIGS. 8(a) through 8(d), the modem control for thereceiving modem is schematically illustrated. The receiving modem checksto determine whether the modem is in use (step 400) and if so, signals areturn (step 402). If the modem is not in use, the application checksfor a new message in the receive buffer (step 404). If a message ispresent in the receive buffer, the contents of the receive buffer arecopied (step 406) to the applications buffer and the line status ischecked for RTS “SEND” and CTS “LOW” (step 408). If these conditions aremet, the RTS line is set “HIGH” (step 410) and the status of the CTSline is checked for a “HIGH” status (step 412).

If the line status of the CTS is “HIGH” the line status in theapplications buffer is set as RTS “ESTABLISHED” and CTS “SEND” (step414). The contents of the application buffer are copied to the transmitbuffer (step 416) and transmitted to the initiating modem (step 418) asa UDP message. A timer is set (step 420) and the receive buffer ischecked for receipt of a message from the transmitting modem (step 422).If a message is received from the transmitting modem before the timertimes out (step 424), the message is copied from the receive buffer intothe applications buffer (step 426).

At this time, the timer has timed out and the number of attemptedtransmission has exceeded the predetermined maximum number of allowableattempted transmissions, (step 428), a error signal is returned to theapplication (step 430). If the timer has not timed out, control isreturned to step 422, or if the timer has timed out and thepredetermined maximum number of attempted transmissions has not beenexceeded, control is returned to step 418.

After the contents of the receiving buffer have been copied into theapplications buffer in step 426, the timer is reset (step 432) and theline status is checked for RTS “ESTABLISHED” and CTS “ESTABLISHED” (step434). Data from the dataline is written to from the dataline on the portto the application (step 436). The status of the CTS is check for “LOW”(step 438) and if this condition is met, the predetermined allowablenumber of attempted transmissions is reset, (step 440) and the CTSstatus is set to “low” in the applications buffer, and the contents ofthe applications buffer is copied to the transmit buffer fortransmission as a UDP message (step 444). a timer is set (step 446) andthe receive buffer is monitored for receipt of a message from thetransmitting modem (step 448). If the timer times out prior to receiptof a message from the transmitting modem, (step 450) the number ofattempted transmissions is compared to the predetermined allowablenumber of attempted transmissions (step 452) and if the maximumallowable number of attempted transmissions is exceeded, an error signalis returned to the application (step 454).

After a message has been received from the transmitting modem, (step448), a copy of the received buffer is copied to the application buffer,(step 456) and the line status is checked for CTS “LOW” and RTS “LOW”(step 458). If these conditions are met, the contents of the transmitbuffer are transmitted (step 460) and a “SUCCESSFUL RECEIVE” message isreturned to the application. If the line status conditions are not met,control is returned to to step 446.

Through the combination of a minimal logic set and minimal signalingdata as set forth above, the present invention provides a transparentwireless link between RS232 interfaces with the same or equivalentreliability as a hard wired link. Further, the present inventionprovides these features with a minimal program size, a feature that isimportant in the applications of interest.

While the present invention has been disclosed and discussed inconnection with the above-described embodiment, it will be apparent tothose skilled in the art that numerous changes, variations andmodifications within the spirit and scope of the invention are possible.Accordingly, it is therefore, intended that following claims shallencompass such variations and modifications.

What is claimed is:
 1. A system for wireless transmission of data,comprising: a first wireless modem operatively connected to a firstserial data interface bearing serial data and a flow control signal andconfigured for encapsulating the serial data and the flow control signalin a wireless carrier signal and transmitting the wireless carriersignal; and a second wireless modem configured for receiving thewireless carrier signal from the first wireless modem, processing thewireless carrier signal to produce the serial data and the flow controlsignal, and transmitting the serial data and the flow control signalacross a second serial data interface, wherein the interfaces are RS232serial data interfaces and the flow control signal includes RTS or CTS.2. A system for wireless transmission of data, comprising: a firstwireless modem operatively connected to a first serial data interfacebearing serial data and a flow control signal and configured forencapsulating the serial data and the flow control signal in a wirelesscarrier signal and transmitting the wireless carrier signal; and asecond wireless modem configured for receiving the wireless carriersignal from the first wireless modem, processing the wireless carriersignal to produce the serial data and the flow control signal, andtransmitting the serial data and the flow control signal across a secondserial data interface, wherein the modems are Cellular Digital PacketData modems.
 3. The system of claim 1, further comprising a datacollection device for collecting the data to be borne by the firstinterface.
 4. The system of claim 1, further comprising a computer forreceiving the data from the second interface.
 5. A system for wirelesstransmission of data, comprising: a first RS232 serial data interfacefor outputting serial data; a first wireless CDPD modem operativelyconnected to the first interface and comprising: means for processingserial data received from the first interface, the data comprising RS232data and RS232 flow control signals, means for encapsulating the dataand the signals in a CDPD courier, and means for transmitting the CDPDcourier; and a second wireless CDPD modem for receiving the CDPD courierfrom the first modem, the second modem being operatively connected to asecond RS232 serial data interface and comprising means for processingthe CDPD courier and transmitting the data across the second interface.6. The system of claim 5, further comprising a data collection devicefor collecting the data to be output by the first interface.
 7. Thesystem of claim 5, further comprising a computer for receiving the datafrom the second interface.
 8. A method of transmitting data, comprising:collecting data to be transmitted; transmitting the data and a flowcontrol signal across a first serial data interface to a first wirelessmodem; encapsulating the data and the flow control signal in a wirelesscarrier signal; transmitting the carrier signal from the first modem toa second wireless modem; processing the carrier signal to obtain thedata and the flow control signal; transmitting the data and the flowcontrol signal from the second modem across a second serial datainterface; and receiving the data from the second interface, wherein thefirst interface is an RS232 serial data interface and the flow controlsignal includes RTS or CTS.
 9. A method of transmitting data,comprising: collecting data to be transmitted; transmitting the data anda flow control signal across a first serial data interface to a firstwireless modem; encapsulating the data and the flow control signal in awireless carrier signal; transmitting the carrier signal from the firstmodem to a second wireless modem; processing the carrier signal toobtain the data and the flow control signal; transmitting the data andthe flow control signal from the second modem across a second serialdata interface; and receiving the data from the second interface,wherein the first modem is a Cellular Digital Packet Data modem.
 10. Amethod of transmitting data, comprising: collecting data to betransmitted; transmitting the data and a flow control signal across afirst serial data interface to a first wireless modem; encapsulating thedata and the flow control signal in a wireless carrier signal;transmitting the carrier signal from the first modem to a secondwireless modem; processing the carrier signal to obtain the data and theflow control signal; transmitting the data and the flow control signalfrom the second modem across a second serial data interface; andreceiving the data from the second interface, wherein the second modemis a Cellular Digital Packet Data modem.
 11. A method of transmittingdata, comprising: collecting data to be transmitted; transmitting thedata and a flow control signal across a first serial data interface to afirst wireless modem; encapsulating the data and the flow control signalin a wireless carrier signal; transmitting the carrier signal from thefirst modem to a second wireless modem; processing the carrier signal toobtain the data and the flow control signal; transmitting the data andthe flow control signal from the second modem across a second serialdata interface; and receiving the data from the second interface,wherein the second interface is an RS232 serial data interface.
 12. Amethod for wireless transmission of data, comprising: transmitting thedata across a first RS232 serial data interface to a first wireless CDPDmodem; encapsulating the data and a flow control signal from the firstRS232 serial data interface in a CDPD courier; transmitting the CDPDcourier to a second wireless CDPD modem; obtaining the data and the flowcontrol signal from the CDPD courier; delivering data and the flowcontrol signal to a second RS232 serial data interface.
 13. The methodof claim 12, wherein the first modem is a Cellular Digital Packet Datamodem.
 14. The method of claim 12, wherein the second modem is aCellular Digital Packet Data modem.
 15. The method of claim 12, furthercomprising collecting the data to be output with a monitoring device.16. The method of claim 12, wherein delivering the data to the secondinterface comprises transmitting the data to a computer.
 17. The methodof claim 12, further comprising checking the validity of the transmitteddata with a cyclic redundancy check.
 18. A wireless modem, comprising:means for processing serial data and a flow control signal from a serialdata interface; means for encapsulating the serial data and the flowcontrol signal in a wireless carrier signal; and means for transmittingthe data, wherein the serial data interface includes an RS232 serialdata interface and the wireless carrier signal includes a CellularDigital Packet Data (CDPD) courier.
 19. A method of transmitting data,comprising: processing serial data and a flow control signal from aserial data interface; encapsulating the serial data and the flowcontrol signal in a wireless carrier signal; and transmitting the data,wherein the serial data interface includes an RS232 serial datainterface and the wireless carrier signal includes a Cellular DigitalPacket Data (CDPD) courier.
 20. A wireless modem, comprising: means forprocessing a carrier signal encapsulating serial data and a flow controlsignal; and means for outputting the serial data and the flow controlsignal to a serial data interface, wherein the serial data interfaceincludes an RS232 serial data interface and the wireless carrier signalincludes a Cellular Digital Packet Data (CDPD) courier.
 21. A method oftransmitting data, comprising: processing a carrier signal encapsulatingserial data and a flow control signal; and outputting the serial dataand the flow control signal to a serial data interface, wherein theserial data interface includes an RS232 serial data interface and thewireless carrier signal includes a Cellular Digital Packet Data (CDPD)courier.
 22. A system for wireless transmission of data, comprising: afirst wireless modem operatively connected to a first serial datainterface bearing serial data and a flow control signal and configuredfor encapsulating the serial data and the flow control signal in awireless carrier signal and transmitting the wireless carrier signal;and a second wireless modem configured for receiving the wirelesscarrier signal from the first wireless modem, processing the wirelesscarrier signal to produce the serial data and the flow control signal,and transmitting the serial data and the flow control signal across asecond serial data interface, wherein the first serial data interfacefurther bears another flow control signal, and the first modem isfurther configured for encapsulating the other flow control signal inthe wireless carrier signal.
 23. A method of transmitting data,comprising: collecting data to be transmitted; transmitting the data anda flow control signal across a first serial data interface to a firstwireless modem; encapsulating the data and the flow control signal in awireless carrier signal; transmitting the carrier signal from the firstmodem to a second wireless modem; processing the carrier signal toobtain the data and the flow control signal; transmitting the data andthe flow control signal from the second modem across a second serialdata interface; receiving the data from the second interface;transmitting another flow control signal across the first serial datainterface; and encapsulating the other flow control signal in thewireless carrier signal.
 24. The method of claims 12, further comprisingencapsulating another flow control signal from the first RS232 serialdata interfaces in the CDPD courier.
 25. A method of transmitting data,comprising: processing serial data and a flow control signal from aserial data interface; encapsulating the serial data and the flowcontrol signal in a wireless carrier signal; and transmitting the data;processing another flow control signal from the serial data interface;and encapsulating the other flow control signal in the wireless carriersignal.
 26. A wireless modem, comprising: means for processing a carriersignal encapsulating serial data and a flow control signal; and meansfor outputting the serial data and the flow control signal to a serialdata interface, wherein the means for processing is further configuredfor processing another flow control signal, and the means for outputtingis further configured for outputting the other flow control signal. 27.A method of transmitting data, comprising: processing a carrier signalencapsulating serial data and a flow control signal; outputting theserial data and the flow control signal to a serial data interface;processing the carrier signal to extract another flow control signal;and outputting the other flow control signal to the serial datainterface.